Song Syllable Order Matters to Skylarks

In the last 50 years experimental behavioural studies have demonstrated fascinating parallels between human language and animal communication across different taxa. Birdsong is a particularly good model for such studies. First, like human language, birdsong is subject to vocal learning in early life and, as a consequence, song composition has geographical variants called ‘dialects’ similar to the variants in human language. Second, birdsong consists of the arrangement of discrete units, called syllables, similarly to the way human language is made up of words formed by the order of phonemes. An important but poorly explored parallel is whether the particular arrangement of syllables in birdsong carries information content. This is a possibility explored by Elodie Briefer (Université Paris-Sud, France and ETH Zürich, Switzerland), Fanny Rybak and Thierry Aubin (Université Paris-Sud and CNRS, Orsay, France) in the present issue.

In a properly controlled experiment in which songs with broken syllable order would be played back to birds, an increase in the response would indicate that birds can perceive the change in syllable order (or syntax) and demonstrate that syllable arrangement carries information content. By contrast, no change or a decrease in the response could be interpreted in several ways. It could mean that the broken syntax is not recognized and therefore carries no information or simply that it is perceived as ‘unnatural’ and is therefore ignored. Earlier studies have shown that transforming a syllable from one dialect into a syllable from another dialect by adding an extra note triggers more aggressive response than the unmodified song. However, all earlier experiments involving broken syntax manipulations have shown no change or a decrease in the response compared to unmodified vocalizations.

Briefer and coauthors chose skylarks as their experimental species because the repertoire of these birds consists of more than 300 syllables and their song is long and complex (Fig. 1). The study was carried out in the skylarks' natural habitat. During the breeding season skylark pairs in adjacent territories form small groups separated from other groups by unsuitable habitat. Males deter intruders with a continuous song, which lasts on average 150 s but could be more than 40 min long. They react weakly to intrusions from neighbouring males and much more aggressively to ‘strangers’ from distant groups. Earlier work by the authors had established that neighbouring males share on average 83% of their syllable repertoire and 71% of their phrase (that is a sequence of syllables) repertoire, and that it is these shared phrases that help males discriminate neighbours from strangers. In the present study the authors used playback experiments to test the hypothesis that the sequential arrangement of syllables within shared phrases contains the crucial information enabling such discrimination.

Figure 1.
A spectrogram of a natural skylark song showing clearly the order and different characteristics of the song's component acoustic units (or ‘syllables’). Photo: Elodie Briefer.